"Green Rust" in the Lab and in the Soil


Oremland, Ronald S, John Stolz, Derek Lovley, SCB Myneni, T. K. Tokunago and G. E. Brown, Jr. 1998. “'Green Rust'in the Lab and in the SoilScience 281: 1111.


In their report “Abiotic selenium redox transformations in the presence of Fe(II,III) oxides,” S. C. B. Myneni et al. state (1, p. 1109) that the reduction of selenium (Se)(VI) by iron (Fe)(II,III) oxides (“green rust”) in an artificially constituted system “provides direct evidence for the formation of reduced Se species in anoxic sediments.” By extrapolating their experiments, done under highly specialized laboratory conditions, to encompass suboxic sediments in general, Myneni et al. appear to challenge a body of evidence which demonstrates that the bulk of Se(VI) reduction in such natural systems is directly mediated by bacteria (for example, through dissimilatory selenate reduction).

The evidence presented by Myneni et al. to support their contention for abiotic reduction in sediments was a selective comparison of their rate constants to some of those in the literature. However, agreement of rate constant data provides only circumstantial evidence and does not constitute a rigorous proof for the underlying mechanism. We concede that some amount of abiotic reduction of Se(VI) can theoretically occur in natural systems. However, Myneni et al. have yet to show conclusively that abiotic Se(VI) reduction actually occurs in recent sediments. Furthermore, they also would have to demonstrate that its quantitative significance is of a magnitude comparable to that achieved by the direct bacterial reduction of this element in such environments.

Geochemistry, Geophysics